1. Field of the Invention
This invention relates to an adaptive differential pulse code modulation (ADPCM) decoder and more specifically to such a decoder wherein each of differential values between two consecutive sampled waveforms is quantized by adaptively changing quantizer step sizes for each sampling period.
2. Description of the Prior Art
An ADPCM coding system is well known in the art, in which differential values of a sampled waveform of a speech signal (for example) are quantized by adaptively changing quantizer step sizes. Another ADPCM of an instantaneous companding type has been proposed which adapts, every sampling period, each quantizer step size.
The detailed explanation of the above is given in the documents entitled "Adaptive quantization with a one-word memory" and "Adaptive Quantization in Differential PCM Coding of Speech" (The Bell System Technical Journal, Vol. 52, No. 7, September, 1973).
A conventional ADPCM decoder of the instantaneous companding type is arranged based on the following equations: EQU g.sub.n =g.sub.n-1 .times.M(I.sub.n) (1) EQU S.sub.n =g.sub.n .times.d.sub.n +S.sub.n-1 ( 2)
Where suffix "n" indicates n-th sampling time, and
g.sub.n : step size multiplier,
I.sub.n : ADPCM code,
M(I.sub.n): control code for quantizer step size multiplier,
S.sub.n : reproduced value, and
d.sub.n : differential value between S.sub.n and S.sub.n-1 and function of I.sub.n.
As seen from the above equations, the previously known ADPCM decoders have required two multiplication processes per time-slot and thus have required complex and expensive circuitry.